1. Field of the Invention
This invention generally relates to protecting moisture-sensitive substrates from moisture; to articles such as a fiber optic cable that contain both a moisture sensitive component and a protectant composition; and to compositions that may be used to protect substrates from moisture and/or corrosion.
2. Description of the Related Art
Transmission cables (e.g., service or power wires/cables) as well as telecommunication devices (e.g., fiber optic cables, telephone cables, terminal blocks, junctions and connections) are often exposed to harsh environmental conditions. Yet, both transmission cables and telecommunication devices are required to maintain uninterrupted mechanical and electrical characteristics despite the surrounding environmental conditions. Historically, transmission cables have been strung above ground between poles. Increasingly, these cables are being buried beneath the ground for aesthetic reasons and to protect them from extremes of temperature, rain, snow, ice, high winds, falling tree limbs and the like. However, even when placed underground, these cables are subject to many environmental stresses including mechanical shock during back filling, moisture, the possibility of water immersion, attack from rodents and exposure to salt and other corrosive materials.
Transmission cables typically contain either a bundle of individually insulated copper wires, or a bundle of fiber optic cables. The copper wires, in particular, are often protected by an inner metal shield, which covers the bundle of wires, and an outer sheath made from plastic or other insulating material. Likewise, a communication cable is generally constructed of an outer plastic jacket and an inner metal core wrap with an annular space there between. See, e.g., U.S. Pat. No. 3,745,321 to Eager, Jr. et al. The core wrap enwraps a plurality of twisted insulated conductors with a filler material, such as petroleum, wax, or other hydrocarbons, located between the conductors. The filler material typically functions as an insulator.
Even though wires and cables are normally well protected, if the protective outer sheath is cut or otherwise broken, water can seep into the wire or cable. Whenever moisture is present in the interior of a cable or wire, it tends, over a period of time, to migrate or flow longitudinally into connections at the splice closures, terminals, or the like. In the special case of optical fibers, passage of the water to connection points or terminals and associated equipment will typically result in damage to such equipment, especially to any metal parts thereof, and can also cause problems at low temperature or freezing environments due to fiber microbending. This is particularly true when an immersed wire is cut.
In an inadequately protected device, corrosion of the contacts or short circuits caused by moisture or other outside elements will eventually interrupt service. Replacement or repair of the cables, whether strung above the ground or buried, can be difficult and/or expensive, and the interruption of service may be particularly irritating to the customer. Because of this, much attention has focused on the protection of these cables from both physical damage and from chemical attack. As a result, various techniques have been disclosed to prevent or restrict migration of moisture along the internal passages of a cable.
In one such technique, cable passages may be filled by a pressurized gas. Upon escape of gas through a ruptured cable jacket, an alarm provides notice of cable damage. However, the gas does not prevent the flow of water into and along the cable internal passages before the repair is effected.
U.S. Pat. Nos. 4,867,526; 5,082,719; and 5,163,115 disclose a super-absorbent polymer or tape which is impregnated with a superabsorbent material for preventing water migration through cables. It has been found that, for a number of reasons, the production and/or installation of these materials is often impractical and/or uneconomical.
Silicone grease lubricant, elastomeric seals, and/or mastic sealing strips have been used in an attempt to seal out moisture. While these sealants offer some protection, moisture can often seep into the enclosure and cause a short circuit, which will interrupt telecommunications service, particularly when the terminal block is buried underground.
It is also known that insulating materials, such as mineral oil, protect the conductor from moisture and corrosive materials. However, these such insulating materials drip when the cable is being serviced (i.e., cut). To overcome this “dripping” problem, various gelling agents are known to be added to the insulator, to thereby provide a gelled, non-dripping protectant composition. See, e.g., U.S. Pat. Nos. 6,169,160; 6,160,939; and 6,085,009.
Zeolites have recently been suggested as a suitable material to impart moisture-resistance to a cable. See, e.g., U.S. Pat. No. 6,205,276 B1.
Hydrophobic fumed silica has been used as a gelling agent, particularly for fiber optic cables. See, e.g., U.S. Pat. Nos. 5,905,833; 5,902,849; 5,737,469; 5,285,513; 5,276,757; and 5,187,763; and European Patent EP 0 206 234 B1. The fumed silica, in combination with a suitable fluid, will cause the fluid to become gelled and/or demonstrate thixotropic properties, which is advantageous for a cable protectant composition. Fumed silica is rather expensive, and so suitable fluids that are relatively less expensive have a commercial advantage.
Due to the relatively long lengths of cable that are typically installed, certain cable components are made from plastic components such as polypropylene in an attempt to both reduce the cost of the cable as well as obtain multiple supply sources for such components. This presents a problem to the supplier of protectant compositions in that many known protectant compositions are not compatible with plastics such as polypropylene and can cause deterioration of those materials. Furthermore, although only a fairly small amount of protectant composition is present in a cross-section of cable, because the cables are extremely long, even a small difference in the per pound cost of the protectant composition can have a large commercial advantage in the marketplace, assuming all other performance properties are met.
A need therefore exists for suitable and cost-effective protectant compositions to be used in transmission cables and telecommunication devices. The present invention fulfills these needs and further provides related advantages.